1. Field of the Invention
This invention relates to a transfer apparatus having a transfer charging member for transferring a toner image from an image bearing member to a transfer material (recording material).
This invention particularly relates to a transfer apparatus in an image forming apparatus such as an electrophotographic copying apparatus, an electrophotographic printer or an electrophotographic facsimile apparatus, and a device for transferring a toner image directly or indirectly formed on the surface of an image bearing member by the use of a charged toner by suitable image forming process means such as electrostatic recording or magnetic recording to the surface of a recording material.
2. Related Background Art
Heretofore, for example, in an image forming apparatus of the electrophotographic type or an image forming apparatus of the electrostatic recording type, a corona transfer apparatus has been used as transfer means for transferring a toner image formed or an image bearing member such as a photosensitive member to a recording material such as sheet. This corona transfer apparatus is disposed a relationship with the recording material and utilizes corona discharge to impart a predetermined charge to the recording material and transfer the toner image on the image bearing member to the recording material, and is effective as a non-contact transfer means. However, it suffers from the problem that the application of a high voltage is necessary for the creation of corona discharge or that ozone is created during corona discharge. Therefore, in recent years, there has been developed an image forming apparatus using transfer means of the contact type capable of transferring an image by the application of a relatively low voltage. This transfer means of the contact type is generally provided with an electrically conductive roller or the like adapted to contact with the back of a recording material, and has a relatively low bias voltage applied thereto to thereby transfer a toner image on an image bearing member to the recording material. Since a low voltage can be applied to the transfer means of such contact type, a power source can be made compact and there is a merit such as a small amount of ozone is created.
FIG. 15 of the accompanying drawings is a schematic construction view of an image forming apparatus of the electrophotographic type for illustrating the prior art.
This image forming apparatus is provided with a photosensitive drum 1 rotatively driven in the direction of arrow Y1, and around it, there are disposed in succession along the direction of rotation thereof a charging roller (charging means) 2 for uniformly charging the surface of the photosensitive drum, an exposure device 3 for forming an electrostatic latent image conforming to image information on the photosensitive drum 1, a developing device 4 for causing a toner to adhere to the electrostatic latent image to thereby form a toner image, a transfer roller (transfer member) 5 for transferring the toner image on the photosensitive drum 1 to a recording material 15, a cleaner for removing any residual toner or the like on the photosensitive drum 1 after the transfer of the toner image, and a fixting device for fixing the toner image transferred onto the recording material 15.
The operation of this image forming apparatus will now be described. The surface of the photosensitive drum 1 is uniformly charged by the charging roller 2, whereafter it is subjected to exposure by scanning light conforming to image information from the exposure device 3 such as a laser scanner, whereby an electrostatic latent image is formed. This electrostatic latent image is developed into a toner image by a toner contained in the developing container 4b of the developing device 4 and carried in a layer-like form on the surface of a developing sleeve 4a. The recording material 15 set on a sheet supply tray 14 is supplied to the nip portion m between the photosensitive drum 1 and the transfer roller 5 by a conveying system including a sheet supply roller 13, register rollers 10, 11, conveyance guides 12a, 12b, transfer guides 8, 9, etc., and the toner image on the photosensitive drum 1 is transferred onto the recording material 15 by the action of the transfer roller. Thereafter, the recording material 15 is separated from the photosensitive drum 1 and is sent to the fixting device 7 through the conveyance guide 12b, whereby the transferred image is fixted and the recording material is discharged onto a sheet discharge tray 16. On the other hand, the photosensitive drum 1 continues its rotation, and adhering materials such as residual toner adhering to the surface thereof are removed by the cleaner 6, and the photosensitive drum is used for the next image forming process.
The above-described transfer apparatus according to the prior art, however, has suffered from the following inconvenience. When in the above-described transfer apparatus, transfer is to be effected onto a recording material of relatively high resistance under an environment of e.g., low temperature (10.degree. C.) and low humidity (10%), there occurs the inconvenience of a slight reduction in the quality of image by bad transfer called "explosion" and "splash" which will hereinafter be described.
The "explosion" is a phenomenon occurring when an sheet chiefly of high resistance is used under an environment of low temperature and low humidity, and is a bad transfer in which for example, on an image sample, a toner is scattered over a white portion around a solid black portion as if it were exploded, thereby reducing the image quality. This is considered to be because that the unfixted toner image once transferred onto the image is subjected to the influence of excess charges on the back of the transfer sheet and they mutually cause repulsion and the toner is scattered as if it were exploded. Also, the "splash" is a phenomenon in which the toner, when transferred, is not transferred to a predetermined position but splashes in any direction, and is bad transfer in which for example, on an image sample, the outline around a character portion becomes blurred to thereby disturb the character image or the like and reduce the quality of image.
Also, particularly in the case of the roller transfer type, it is necessary to go through a complicated process in the manufacture of a member and the cost becomes high and thus, a more inexpensive transfer apparatus is required.